Objective.



corrections for spherical and astigm atic errors EDWRDv BAUSCH, OF ROCHESTER, NEW YORK, ASSIGNOR TO T it LOMB OPTICAL COMPANY, OF SAME PLACE.

OBJ ECTIVE.

SPECIFICATION forming pm of Letters Patent No. 660,747, dated october 30, ieoo.

Application liled May 31. 1900.

for the spherical, `chromatic, and astigmatic aberrations by more simple means thanhave prevailed heretofore.

Itis a well-known fact that the light-giving properties or so-called speed in a photographic objective depends upon the size of its opening (diaphragm) and the light-transmitting properties of the several lenses of which itA is composed. The mere quality of speed in an objective has little value unless it is accompanied by freedom from spherical, chromatic, and'astigmatic errors, and to accomplish this end an aggregation of lenses have been required, ranging as high as teu' sis lenses in one objective. It followsl that the larger number of lenses involve loss of light by absorption, add weight, and increase cost. Itis also customary to make some of the lenses v of considerable thickness and to use several dispersing and collecting lenses in one combination. The dispersing-lenses are generally of flint-glass of considerable density, which absorbs more light and is more susceptible to atmospheric influence than is the case in `crown-glass. Having these difficulties in mind, I have constructed a lens composed of only four members, one of them only beinga dispersing-lens composed of flint-glass, the others being crown-glass (collecting-lenses) of dilerent refractive indices and all of them being relatively thin and so designed as to be free from spherical, chromatic, and astigmatic errors and having relatively great speed. The several members may beof different curvatures and may be separated by an airspace or may be cemented together. However, I prefer to separate them in order that Serial No. 18,604. (No model.)

may be made, which would be dimcunfof as! complishment in such a high degree if the elements were cementedr In the drawings, Figures 1, 2, and-3 show 55 sectional viewsv of diferent forms of objecltivesconstructedA in accordance with my in,`

vention.

Referring particularly to Fig. 1, the objecw tive is composed of one member or lens on 6o" one side of the usual space provided for the g diaphragm and three on the other, the single lens a being positive or collective and com-y posed of crown-glass and in='the presentinstance having arefractive index of, say, 1.51,1 65

while the positive members c and d are each composed of crown-glass, the former havingr a refractive index of, say, 1.58 and the latter having a relatively higher index, say 1.60. v The single negative or dispersive lens-b is o composed of flint-glass having a refractive index which is higher than that of the positivel lens having the lowest index and lower than that of the positive lens having the highest and is different from the index of the remain! 75 ing positive lens. In the present varrangement thev refractive index of the lens b is 1.56 or between 1.60 and 1.58; but this I do not regard as essential, as it might be 1.54 for instance. 8o

In Fig. 2 I have shown another form of objective embodying myimprovements,in which the members are arranged in systems composed ofpairs separated by air-spaces, the

lenses e and f being of crown-glass having indices of 1.51 and 1.58, respectively, and the other pair being composed of the negative lens g, of fiint, with an index of, say, 1.56, and the positive crown-lens h having an index of, say, 1.60.

In Fig. 3 is shown still another form of objective embodying two similar systems, each embodying lenses having the characteristics of the other forms, excepting that their curvatures are so related that they may be and are preferably cemented together. In this embodiment the positive or collective lens j, of crown-glass, has a refractive index of 1.60, the lens k of 1.58, and the third positive lens m one of 1.51, while the negative lens l, of roo Hint-glass, has an index of 1.56. v The various curvatures of the lenses of all HE BAUsoH- the fornis are so related as to give the desired focus to the objective and correct the spher ical, chromatic, and astigmatic variations in the manner well known to those skilled in the art. v

I claim as my inventionl. An objective composed of fonrlenses having different curvatnres, three of said lenses being positive and each having different refractive indices, and the fourth lens being negative and having a refractive index lying between that of the positive lens having the lowest index and either of the other positive lenses.

2. An objective composed of four separated lenses having different curvatnres, three of said lenses being positive and each having different refractive indices, and the fourth lens being negative vandgha'ving af. refractive i index lying between that of the positive-lens zj havingthe lowest index and either of the other;

positive lenses. An objective composed of four lenses,j one of which is negative, the other three being positive, the refractive index of one of 25 positive lens having the highest index and 3c that of the negative lens.

EDWARD BAUSCH.

Witnesses:l

G. A. RODA, G. WILLARD RICH. 

